1. Field of the Invention
The invention relates generally to earth-boring bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. More particularly, the invention relates to rolling cone rock bits and to an improved cutting structure for such bits. Still more particularly, the invention relates to enhancements in cutter elements and in manufacturing techniques for cutter elements, rolling cone cutters and drill bits.
2. Description of the Related Art
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by revolving the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole formed in the drilling process will have a diameter generally equal to the diameter or “gage” of the drill bit.
A typical earth-boring bit includes one or more rotatable cone cutters that perform their cutting function due to the rolling movement of the cone cutters acting against the formation material. The cone cutters roll and slide upon the bottom of the borehole as the bit is rotated, the cone cutters thereby engaging and disintegrating the formation material in its path. The rotatable cone cutters may be described as generally conical in shape and are therefore referred to as rolling cones.
Rolling cone bits typically include a bit body with a plurality of journal segment legs. The rolling cones are mounted on bearing pin shafts that extend downwardly and inwardly from the journal segment legs. The borehole is formed as the gouging and scraping or crushing and chipping action of the rotary cones remove chips of formation material which are carried upward and out of the borehole by drilling fluid which is pumped downwardly through the drill pipe and out of the bit.
The earth disintegrating action of the cone cutters is enhanced by providing the cone cutters with a plurality of cutter elements. Cutter elements are generally of two types: inserts formed of a very hard material, such as tungsten carbide, that are typically press fit into undersized apertures in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the rolling cone. Bits having tungsten carbide inserts are typically referred to as “TCI” bits, while those having teeth formed from the cone material are commonly known as “steel tooth bits.” In each instance, the cutter elements on the rotating cone cutters breakup the formation to form new borehole by a combination of gouging and scraping or chipping and crushing.
In oil and gas drilling, the cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipes, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. Accordingly, it is always desirable to employ drill bits which will drill faster and longer, and which are usable over a wider range of formation hardness.
The length of time that a drill bit may be employed before it must be changed depends upon its ability to “hold gage” (meaning its ability to maintain a full gage borehole diameter), its rate of penetration (“ROP”), as well as its durability or ability to maintain an acceptable ROP. The form and positioning of the cutter elements in the cone cutters greatly impact bit durability and ROP and thus, are critical to the success of a particular bit design.
The inserts in TCI bits are typically inserted in circumferential rows on the rolling cone cutters. Most such bits include a row of inserts in the heel surface of the cone cutters. The heel surface is a generally frustoconical surface and is configured and positioned so as to align generally with and ream the sidewall of the borehole as the bit rotates. In addition to the heel row inserts, conventional bits typically include a circumferential gage row of cutter elements mounted adjacent to the heel surface but oriented and sized so as to cut the corner of the borehole. Conventional TCI bits also include a number of additional rows of cutter elements that are positioned in circumferential rows disposed radially inward or in board from the gage row. These cutter elements are sized and configured for cutting the bottom of the borehole, and are typically described as inner row cutter elements.
A variety of different shapes of cutter elements have been devised. In most instances, each cutter element is designed to optimize the amount of formation material that is removed with each “hit” of the formation by the cutter element. At the same time, however, the size, shape and design of a particular cutter element is also dependent upon, and many times compromised by, factors such as the location in the drill bit in which it is to be placed, the type of formation, and the element's vulnerability to the forces expected to be encountered.
TCI inserts generally include a cylindrical barrel or base portion that is embedded and retained within a cylindrical hole or bore formed in the cone steel, and a cutting portion that extends above the cone steel for engaging the formation material. To retain an insert in the cone, a predetermined barrel length is typically required for a given diameter and length of insert. In certain bit designs, and at particular locations on the rolling cone, it may be desirable to provide an insert having a cutting portion with a relatively large cutting surface so as to enhance the removal of the formation material at the locations where that cutter element insert engages the formation. Unfortunately, it is many times impossible to provide an insert with the cutting portion of the desired size due to limitations in the core steel available for retaining the insert's base. More particularly, bores formed in the cone steel for retaining other inserts in the same row, as well as bores retaining inserts in other rows in the cone, limit the depth and diameter of a given hole. The various adjacent holes must be separated to the extent such that the steel in the region has sufficient strength to retain the insert when it undergoes the extreme forces imparted by the formation as the bit is rotated in the borehole, such forces including both impact forces and forces tending to bend or rotate the insert. In short, the limited volume of cone steel available for receiving and retaining the base portion of inserts has typically limited the size and shape of the cutting portion of the insert. Accordingly, in order to design a bit that produced acceptable ROP and reasonable durability, compromises had to be made in the size and shape of the inserts.
In an attempt to provide a larger cutting portion, certain conventional inserts have been made that extended beyond the footprint or envelope of the base portion of the insert. Examples of such inserts include those described as being formed with a negative draft as shown in U.S. Pat. No. 6,241,034, incorporated herein by reference. While providing the advantage of an increased cutting surface area, as compared to other conventional inserts, such inserts are more expensive to manufacture and are difficult to secure in the cone in a way that prevents rotation of the insert and misalignment of the cutting portion with the desired orientation.
In U.S. Pat. No. 5,421,423, incorporated herein by reference, inserts having elongate cutting portions and correspondingly-shaped elongate base portions are disclosed. Such inserts are described as being press fit into elongate slot-shaped sockets formed in the cone steel, where such slots are formed by boring spaced apart holes in the cone steel and then milling the steel between the two bores to form a slot having the same width as the diameter of the bores. This method of forming the slotted socket thus requires machining that, relative to merely boring holes into the cone steel, is more time consuming, expensive, and exacting. Providing a slot-like socket capable of retaining the elongate, non-circular insert by interference fit is difficult to achieve.
Accordingly, to provide a drill bit with higher ROP and better durability, and thus to lower the drilling costs incurred in the recovery of oil and other valuable resources, it would be desirable to provide cutter elements having desirably shaped and sized cutting portions that have larger cutting surfaces than those that can be retained in a conventional aperture. Further, it would be advantageous that such cutter elements resist the rotation and movement within the aperture and be retained in the cone steel even in instances where the cone steel is limited in both cone surface area and depth of permissible bore. Preferably, such cutter elements and the methods for manufacturing cone cutters and bits would provide a bit that will retain cutting inserts and protect the cone steel for longer periods than conventional methods and apparatus so as to yield improved ROPs and an increase in footage drilled.